The present invention relates to a seal pin indexed precisely in a resin transfer molding die to assure the desired resin-to-fiber ratio in the mold part, and, thereby, to produce reliably and consistently a part of minimum weight with adequate strength.
Reducing the cost of aerospace parts requires a switch to reliable, repeatable processes that yield parts, which meet the rigorous requirements of aerospace. In aerospace, the issues of absolute weight, shape, strength, and durability must be traded during the design of a part. A factor often ignored during this design trade is the cost of manufacture. Often, however, safety forces part designs that are expensive to manufacture. Resin transfer molding (RTM) is promising as a process to replace complicated built-up metal assemblies with one-piece fiber-reinforced resin composite alternatives. One challenge with RTM is ensuring that the resin-to-fiber ratio in the molded part is adequate everywhere in the part so that the molded part will have the design strength and durability. The present invention is an improvement in RTM using a seal pin in the mold die to index the preform for the part reliably and precisely on the mold cavity on an inner mandrel that is completely sealed within the die set.
Resin transfer molding (RTM) is a process by which a resin is pumped at low viscosities and low pressures into a closed mold die set containing a preform of dry fabric to infuse resin into the preform and to make a fiber-reinforced composite part. The RTM process can be used to produce at low cost composite parts that are complex in shape. These parts typically require continuous fiber reinforcement along with inside mold line and outside mold line controlled surfaces. The ability to include and place continuous fiber reinforcement in large and small structures sets RTM apart from other liquid molding processes.
Some RTM mold die sets require outer dies to define a cavity and an inner mandrel that seats in the cavity to define a hollow region in the molded part. Often the mandrel must be assembled from multiple parts so that it can be disassembled and removed from the molded part. RTM die sets that used mandrels usually are configured so that the mandrel is inserted through an opening in the dies. Sealing the base plate of the mandrel with the opening in the dies was challenging. With the seal/index pins of the present invention, the mandrel can be positioned entirely within the mold cavity, allowing a simpler seal system between the dies.
The present invention relates to a seal pin indexed precisely in a resin transfer molding die to assure the desired resin-to-fiber ratio in the mold part, and, thereby, to produce reliably and consistently a part of minimum weight with adequate strength. The index/resin seal pin of the present invention for RTM mold dies provides a seal (via replaceable O-rings) on the threaded stem of the pin to prevent low viscosity resin from invading the threads, bolts pins, or other elements of the assembled parts that form the inner mandrel. The pin allows the entire inner mandrel to be enclosed completely and sealed within the mold cavity, simplifying the sealing system. Preventing resin flow into the mandrel""s parts allows their easy disassembly following injection of the resin and molding of the part without causing damage to the part. The pin seats in a complementary recess in the dies to provide close tolerance indexing of the mandrel in the mold cavity. Without this indexing, the mandrel would not position the fiber preform accurately within the mold cavity. Following injection of the resin and cure, the molded part could have areas starved of resin or areas too rich in resin. Starved areas would have insufficient strength while rich areas introduce unnecessary weight.
The RTM process initially evacuates the mold cavity, exposing the mandrel to a high vacuum. Resin injection follows with an injection and cure pressure of up to 300 psi. Therefore, proper sealing is a critical feature for obtaining quality parts.
The present invention relates to the seal/index pin for the inner mandrel, to dies for accommodating the pin-mandrel assembly, and to RTM methods for using the seal/index pins on a mandrel completely enclosed within the mold cavity to mold composite parts.